The invention relates to the manufacture of fibrous webs in which a foamed fiber-containing slurry is deposited on a moving support to form a continuous web that is further treated to form a product such as tissue paper.
In a common method of manufacturing a paper web, an aqueous slurry (furnish) of wood and/or other fibers is discharged through the outlet (slice or slice opening) of a distributor (headbox) onto a continuously moving foraminous support (Fourdrinier wire) or between facing surfaces of two such moving supports in the form of a continuous fibrous web. This web is dried and subjected to subsequent treatments to form the final paper product. The headbox can provide a single jet, or several jets of the same or different furnishes which may or may not merge into a single jet by the time they reach the moving support.
One of the important factors in this process is control over the slurry jet or jets discharged from the slice or slices, including control over the jet velocity. Such control can influence significantly important properties of the resulting paper product such as the orientation of the fibers in the web and, consequently, properties such as the tensile ratio of the paper product (longitudinal vs. transverse tensile strength). Such control can be important both in the case of single slice headboxes used to make paper products such as tissue as well as in the case of multiple slice headboxes used to make similar products or stratified products such as webs having a bulk-providing central stratum sandwiched between thinner but stronger outer strata.
When a simple slurry of fibers in a liquid is used, control over the jet can be easier than in the case of a foamed slurry where the vehicle in which the fibers are dispersed contains both liquid and gas phases, e.g., when the vehicle is a dispersion of air bubbles in water containing a suitable surfactant. See U.S. Pat. Nos. 4,764,253, 4,086,130 and 3,716,449, which are hereby incorporated by reference. See, also, an advertisement by the Beloit Corporation of Wisconsin in TAPPI, February, 1990 for a headbox producing a jet in the form of multiple thin converging layers. A foamed slurry can be advantageous, e.g., when the slurry contains fibers which have been rendered anfractuous (kinked) by a process such as milling in order to enhance properties of the final product such as bulk and softness. A conventional water furnish tends to relax the fiber kinks at too high a rate but a foamed furnish tends to reduce the exposure of the fibers to water and to maintain their desired anfractuous properties.
The unique properties of foamed furnish with respect to fluid parameters such as density, viscosity, compressibility and effects of temperature and pressure, can introduce significant difficulties in the control over the headbox jet or jets. Because relevant properties of foamed furnish are so dependent on parameters such as air content and temperature, strategies for controlling jet velocity that may be suitable for a conventional slurry of fibers in a liquid may not be appropriate for foamed furnishes. One type of control suitable for foamed furnishes is discussed in said U.S. Pat. No. 4,764,253 and uses the output of a magnetic flowmeter and knowledge of the headbox and the slice cross-sectional areas to calculate a control signal for a pump delivering the furnish to the headbox. Properties of frothy liquids are discussed in Van Dyke, M., et al., Annual Review of Fluid Mechanics, Vol. 4, pp.369-396, 1972 (see, in particular, page 384), but not in the context of paper making. Accordingly, it is believed that a need still remains for a simpler, less expensive and more reliable and effective control, and this invention is directed to meeting such a need.
In one non-limiting example, the invention is embodied in system which uses only measurements of the pressure and density of the furnish using neither direct measurement of flow velocity nor direct measurement of volume flow rate of the furnish jet, which are easily and reliably obtained, to calculate the jet velocity and to control the delivery of furnish to the headbox so as to move the calculated jet velocity toward a target velocity. This can be done in accordance with the invention on the basis of comparing the calculated and target jet velocities or on the basis of comparing a calculated headbox pressure with a target headbox pressure.
In particular, an exemplary embodiment of the invention periodically measures the density of the foamed furnish fed to the headbox, for example with a radioactive mass sensor, and the pressure of the furnish at two points, one before the headbox and another in the headbox. A computing circuit uses these measurements to estimate. (e.g., calculate) the current velocity of the jet emitted from the headbox. The system then compares the estimated current jet velocity with a target velocity which typically is selected on the basis of the machine or wire speeds. The result of this comparison controls the feeding of furnish to the headbox to reduce the difference between the estimated and target jet velocities. In the alternative, the system can use the measurements to estimate (e.g., calculate) a target headbox pressure and can control the feeding of furnish to bring the current measured headbox pressure closer to the estimated target headbox pressure.
The invented system recognizes the influence of the air content and headbox pressure on jet velocity and can optionally use at least one empirical correction factor to enhance the control over jet velocity or headbox pressure. The relationships between measurements and controlled parameters which the invention uses are believed to be particularly efficacious in accounting for the unique properties of foamed furnish in paper making. The invention is believed to be useful for single jet systems, for systems using multiple jets of the same furnish and for systems using stratified jets of different furnishes. One preferred use of the invention is in a paper making line using foam forming and surfactant recovery techniques discussed in copending commonly owned patent applications Ser. No. 07/599,149 filed on Oct. 17, 1990 in the name of John H. Dwiggins and Dinesh M. Bhat and entitled Foam Forming Method and Apparatus, now abandoned and Ser. No. 07/598,995 filed on Oct. 17, 1990 in the name of Dinesh M. Bhat and entitled Recovery of Surfactant From Paper Making Process, now abandoned. However, the invention is not limited to use in such a paper making line.